Sensors, actuators/controlling devices (hereinafter “controllers”), and alarms are used for a variety of purposes and in many different applications, both industrial and consumer. There are many different types of sensors, controllers and alarms. And some of such devices have a singular function (i.e., sensing, controlling, or alarming). Others perform multiple duties, such as sensing and controlling, sensing and alarming, or sensing and alarming and controlling.
These devices are typically positioned at a variety of locations throughout a system in order to acquire data, take or control some action, or provide an alarming function. Usually, the devices are at “remote” locations; that is, they are sited at a location that is remote from a central monitoring area, etc.
Regardless of their functionality, almost all of these remotely-located devices require a power supply to operate properly. Sensors require power to change a native signal that they measure into a more conveniently-transmittable signal. Controllers and alarms require significantly more power than is usually available in the transmitted signal to either control the media or create an alert signal.
The current state of power requirements for sensors, controllers, and alarms are summarized as follows.
SENSORS: a few examples of sensors include current sensors, energy sensors, flow sensors, humidity sensors, light sensors, particle sensors, pressure sensors, proximity sensors, radiation sensors, temperature sensors, velocity sensors, voltage sensors, weight sensors.
Two important purposes of sensors are (1) to determine a specific characteristic of matter and (2) to transmit information regarding this characteristic to another device (e.g., a central processing system or a controller/alarm, etc.).
Typically, the matter being measured is a solid, liquid, gas, or mixtures thereof. The characteristic being measured is often a property of the matter, such as temperature, flow, or weight. But a sensor might alternatively be measuring a more intangible quality, such as, without limitation:                The amount of a particular chemical in a certain volume of air.        The speed of a particle relative to another particle.        The amount of radiation being emitted from an item.        The intensity of light landing on a surface.        The amount of a substance passing by or through a location.        
Sensors utilize various methods of transforming the measured quantity into a transmittable signal. They all start with a “native” measuring characteristic, then often change this to a “transmittable” characteristic if the native characteristic is not well suited for transmission to the monitoring system and/or controller and/or alarm.
Typically, signal(s) from a sensor are converted into a voltage, current, or frequency output and conveyed over a transmission medium (e.g., wires, fiber, etc.) to a processing system or other device. The signal(s) can alternatively be transmitted wirelessly via various methods, such as by using radio waves, microwaves, light waves, sound waves, and the like. In some cases, the native signal is not converted; rather, it is amplified for transmission. In some cases, the signal is digitized for transmission.
This conversion from “native” to the “transmittable” signal requires a power source of some type, which is situated either local to the sensor or remote from it. A few examples follow to demonstrate certain issues pertaining to native and transmittable signals.